Turkish investigators probing the vanishing of a Saudi journalist in Istanbul believe he was murdered inside the Saudi consulate, Turkish sources told news agencies on Saturday.

A senior Turkish police source told the Middle East Eye that police believed that Jamal Khashoggi, 59, was "brutally tortured, killed and cut into pieces" inside the consulate after visiting the building on 2 October.

"Everything was videotaped to prove the mission had been accomplished and the tape was taken out of the country," the source told the MEE.

On Tuesday he entered the Saudi consulate in Istanbul to get documents for his forthcoming marriage. Saudi officials say he left shortly afterwards but his fiancee, who was waiting outside, said he never came out.

Khashoggi, a former newspaper editor in Saudi Arabia, left the country last year. He advised Prince Turki al-Faisal, former Saudi intelligence chief, and has also been close to billionaire investor Prince Alwaleed bin Talal.

He interviewed Al Qaeda leader Osama bin Laden several times in Afghanistan and Sudan.

In his columns in the Washington Post, Khashoggi had been escalating his criticism of Crown Prince Mohammed bin Salman.

Speaking to CNN Turk on Sunday, Yasin Aktay, an adviser to Turkish President Recep Tayyip Erdoğan, said: "We could determine his entrance but not any exit. That's confirmed. We asked them [the Saudis], they say 'he left,' but there is no such thing on the camera footage.

Aktay said he believed Khashoggi had been killed in the consulate and that Turkish authorities believed a group of 15 Saudi nationals were "most certainly involved" in the matter.

An unnamed Turkish official told the Reuters news agency that Turkish police believed Khashoggi had been killed and his body then removed from the building.

"The initial assessment of the Turkish police is that Mr Khashoggi has been killed at the consulate of Saudi Arabia in Istanbul. We believe that the murder was premeditated and the body was subsequently moved out of the consulate," said the official.

The Washington Post said Saturday, citing sources familiar with the inquiry, that its columnist Khashoggi had been killed by a 15-man Saudi team sent to Turkey "specifically for the murder."

Police said about the 15 Saudis, including officials, came to Istanbul on two private flights on Tuesday and were at the consulate at the same time as Khashoggi. They left again the same day. Their diplomatic bags could not be opened.

Khashoggi's disappearance is unlikely to further deepen divisions between Ankara and Riyad. Erdoğan on Sunday said he was awaiting the results of an investigation

Erdoğan said on Sunday he was closely following the case of Khashoggi. Erdoğan told reporters that authorities were examining camera footage and airport records as part of their investigation into the disappearance last week of Khashoggi.

The Turkish president said he was personally following the issue, without saying what he believed had happened to Khashoggi.

"Entries and exits into the embassy, airport transits and all camera records are being looked at and followed. We want to swiftly get results," he said, adding without explanation: "My expectation is still positive."